Development of internal damage under compression tests was evaluated in order to predict crack initiation and fracture. Moreover, three-point bending tests were carried out in order to clarify deformation and fracture under tensile stress state in comparison with those under compressive stress state. Columnar specimens for compression tests and square pole specimens for three-point bending tests were prepared by using clinical resin composites. In compression tests, loading-unloading-reloading was given to columnar specimens and Young's modulus was evaluated by the gradient of stress-strain curves at unloading. Internal damage was evaluated from the variation of Young's modulus. The variation of density and residual strain were also discussed in association with the development of internal damage. Accumulation of internal damage was found on the stress-strain curve under loading-unloading-reloading in comparison with the curve under monotonic loading. On the other hand, in three-point bending tests, dependence of stress-strain curves on strain rate was clarified. Compression tests have been carried out under similar experimental conditions by authors so far. Mechanical behaviors of resin composites under tensile stress state were discussed in comparison with those under compressive stress state. Brittleness under tensile stress state was indicated in comparison with compressive stress state.
